Using a heartbeat signal to maintain data consistency for writes to source storage copied to target storage

ABSTRACT

Provided are a method, system, and program for using a heartbeat signal to maintain data consistency for writes to source storage copied to target storage. A copy relationship associates a source storage and target storage pair, wherein writes received at the source storage are transferred to the target storage. A determination is made whether a signal has been received from a system within a receive signal interval. A freeze operation is initiated to cease receiving writes at the source storage from an application in response to determining that the signal has not been received within the receive signal interval. A thaw operation is initiated to continue receiving write operations at the source storage from applications after a lapse of a freeze timeout in response to the freeze operation, wherein after the thaw operation, received writes completed at the source storage are not transferred to the target storage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method, system, and program for usinga heartbeat signal to maintain data consistency for writes to sourcestorage copied to target storage.

2. Description of the Related Art

Disaster recovery systems typically address two types of failures, asudden catastrophic failure at a single point in time or data loss overa period of time. In the second type of gradual disaster, updates tovolumes may be lost. To assist in recovery of data updates, a copy ofdata may be provided at a remote location. Such dual or shadow copiesare typically made as the application system is writing new data to aprimary storage device. Different copy technologies may be used formaintaining remote copies of data at a secondary site, such asInternational Business Machine Corporation's (“IBM”) Extended RemoteCopy (XRC), Coupled XRC (CXRC), Global Copy, and Global Mirror Copy.These different copy technologies are described in the IBM publications“The IBM TotalStorage DS6000 Series: Copy Services in OpenEnvironments”, IBM document no. SG24-6783-00 (September 2005) and “IBMTotalStorage Enterprise Storage Server: Implementing ESS Copy Serviceswith IBM eServer zSeries”, IBM document no. SG24-5680-04 (July 2004).

In data mirroring systems, data is maintained in volume pairs. A volumepair is comprised of a volume in a primary storage device and acorresponding volume in a secondary storage device that includes anidentical copy of the data maintained in the primary volume. Primary andsecondary control units, also known as storage controllers or enterprisestorage servers, may be used to control access to the primary andsecondary storage devices. In certain backup system, a sysplex timer isused to provide a uniform time across systems so that updates written bydifferent applications to different primary storage devices useconsistent time-of-day (TOD) value as a time stamp. Application systemstime stamp data sets when writing such data sets to volumes in theprimary storage. The integrity of data updates is related to ensuringthat updates are done at the secondary volumes in the volume pair in thesame order as they were done on the primary volume. The time stampprovided by the application program determines the logical sequence ofdata updates.

In peer-to-peer remote copy operations (PPRC), multiple primary controlunits may have source/target pairs, i.e., volume pairs, included inconsistency groups so that data copied to target volumes by thedifferent primary control units maintains data consistency. A hostsystem includes a program, referred to as a consistency manager, tomaintain data consistency across the different primary control unitshaving source/target pairs in a consistency group. In the current art,if a primary control unit detects an error, such as a failure with theconnection to secondary control unit managing access to the targetstorage in the source/target pair, then the primary control unit mayinitiate a freeze operation to block any further writes to the sourcevolumes. In response to the freeze operation, application programsblocked from writing data would not write any more data to any primarycontrol unit. After initiating the freeze operation, the primary controlunit would send an interrupt to the consistency manager identifying thefreeze and set a freeze timeout timer. At the expiration of the freezetimeout timer, the primary control unit would initiate a thaw operationto start accepting writes from the application to the source storage inthe source/target pair, but not copy the writes to the target storage.

In the current art, if the primary control unit cannot communicate theinterrupt to the consistency manager to allow the consistency manager tosend freeze commands to all primary control units, then applicationswriting to primary control units other than the primary control unitwhere the freeze occurred may have their data writes transferred to thetarget storage even though data at the primary control unit where thefreeze occurred would not copy writes to the target storage. This mayresult in data inconsistency at the target storage.

For these reasons, there is a need in the art to provide techniques formaintaining data consistency.

SUMMARY

Provided are a method, system, and program for using a heartbeat signalto maintain data consistency for writes to source storage copied totarget storage. A copy relationship associates a source storage andtarget storage pair, wherein writes received at the source storage aretransferred to the target storage. A determination is made whether asignal has been received from a system within a receive signal interval.A freeze operation is initiated to cease receiving writes at the sourcestorage from an application in response to determining that the signalhas not been received within the receive signal interval. A thawoperation is initiated to continue receiving write operations at thesource storage from applications after a lapse of a freeze timeout inresponse to the freeze operation, wherein after the thaw operation,received writes completed at the source storage are not transferred tothe target storage.

In an additional embodiment, there is information on multiple sourcestorage and target storage pairs maintained by control units, whereinthe control unit maintaining the pair copies writes to the sourcestorage to the target storage. A determination is made of freezetimeouts used by control units maintaining the source and target pairs.In response to a freeze operation with respect to one source and targetpair managed by one control unit, the control unit blocks writes to thesource storage. The control unit initiates a thaw operation to continuereceiving write operations at the source storage after a lapse of thefreeze timeout for the source and target pair in response to the freezeoperation. After the thaw operation, received writes completed at thesource storage are not transferred to the target storage. Adetermination is made of a send signal interval based on the determinedfreeze timeouts. A signal is communicated at the send signal interval tothe control units maintaining the source and target pairs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a network computing environment.

FIG. 2 illustrates an embodiment of information maintained for a copyrelationship.

FIG. 3 illustrates an embodiment of copy relationship and otherinformation maintained at the primary control units.

FIG. 4 illustrates an embodiment of session information.

FIG. 5 illustrates an embodiment of consistency group information.

FIG. 6 illustrates an embodiment of primary control unit information.

FIG. 7 illustrates an embodiment of operations to register source/targetpairs maintained by primary control units to include in a consistencygroup.

FIG. 8 illustrates an embodiment of operations performed by aconsistency manager to send heartbeat signals to primary control units.

FIG. 9 illustrates an embodiment of operations for a primary controlunit to monitor for heartbeat signals from a consistency manager tomaintain data consistency.

DETAILED DESCRIPTION

FIG. 1 illustrates an embodiment of a network computing environment. Anetwork 2 includes a plurality of primary control units 4 a . . . 4 n;primary storages 6 a . . . 6 n; secondary storages 12 a . . . 12 n; ahost 14 writing updates to the primary storages 6 a . . . 6 n; and aconsistency manager 16 maintaining data consistency among source andtarget storage pairs managed by the primary 4 a . . . 4 n and secondary10 a . . . 10 n control units. The components 4 a . . . 4 n, 6 a . . . 6n, 12 a . . . 12 n, 14, 16, and 18, are connected to the network 2 andthe network 2 enables communication among these components. The network2 may include one or more switches to provide one or more paths ofcommunication between the different network 2 elements.

The primary 4 a . . . 4 n and secondary 10 a . . . 10 n control unitsinclude copy manager software 20 a . . . 20 n and 22 a . . . 22 n,respectively, that manages the copying of writes to locations in theprimary storages 6 a . . . 6 n in a source/target copy pair to targetstorage 10 a . . . 10 n indicated in the source/target copy pairinformation. The primary copy manager 20 a . . . 20 n may read updatesfrom the primary storages 6 a . . . 6 n and send the writes to theprimary control unit 4 a . . . 4 n that manages the copying of thewrites in the order in which they were written to the primary storages 6a . . . 6 n to the corresponding secondary storage 12 a . . . 12 n(target). The dependent order of the writes may be maintained by writingthe data synchronously, so that the data will be on the target andsource storage before the application 24 is allowed to proceed with anext write. Therefore, the data will be consistent on the targets as aresult of the application 24 using ordered dependent writes for datathat needs to be consistent with itself. Thus, when data is recoveredfrom the target storage, i.e., secondary storage 12 a . . . 12 n, therecovered data will be consistent.

The copy managers 20 a . . . 20 n, 22 a . . . 22 n may copy data bysending the writes to the primary control units 4 a . . . 4 n, whichthen manage and initiate the synchronously copying from the source tothe storage using a technique such as peer-to-peer remote copy (PPRC).Complete may be returned to the application 24 providing the writes uponcompleting the write at the primary control unit 4 a . . . 4 n or thesecondary control unit 10 a . . . 10 n. Alternatively, the primarycontrol units 4 a . . . 4 n may copy data asynchronously using remotecopy technology.

The consistency manager 16 maintains consistency across storage/targetpairs managed by primary control units 4 a . . . 4 n. Each primarycontrol unit 4 a . . . 4 n includes information on one or more copyrelationship, each copy relationship specifying source locations in theprimary storage 6 a . . . 6 n, e.g., LSSs, volumes, etc., copied tocorresponding target locations in the secondary storage 12 a . . . 12 n.

The network 2 may comprise a Storage Area Network (SAN), Local AreaNetwork (LAN), Intranet, the Internet, Wide Area Network (WAN),peer-to-peer network, arbitrated loop network, etc. The storages 6 a . .. 6 n, 12 a . . . 12 n may comprise an array of storage devices, such asa Just a Bunch of Disks (JBOD), Direct Access Storage Device (DASD),Redundant Array of Independent Disks (RAID) array, virtualizationdevice, tape storage, flash memory, etc.

The consistency manager 16 may be implemented within one of the primaryor secondary control units or in a separate system, such as shown inFIG. 1.

FIG. 2 illustrates an embodiment of copy relationship informationmaintained by the copy managers 20 a . . . 20 n and, in certainembodiments, 22 a . . . 22 n. Each copy relationship 50 instanceincludes: a copy relationship identifier (ID) 52; the source storage 54locations, e.g., LSS, in the primary storages 6 a . . . 6 n involved inthe copy relationship; the corresponding target storage 56 locations inthe secondary storages 12 a . . . 12 n to which writes to the sourcestorage 54 locations are copied; and a freeze timeout 58 for the copyrelationship 52, e.g., LSS pair. For instance, if a freeze operation isperformed at the primary control unit 4 a . . . 4 n due to some error,then after the freeze timeout time 58 has elapsed for the particularcopy relationship 52, the primary control unit 4 a . . . 4 nautomatically initiates a thaw operation to start accepting writes tothe source storage 54 locations from the application 24 without copyingthe writes to the corresponding target storage 56 location. In oneembodiment, the copy manager 20 a . . . 20 n may issue the thaw to theprimary control unit 4 a . . . 4 n before the timeout time if the copymanager 20 a . . . 20 n determines that all the source LSS pairs havebeen frozen to ensure data consistency. In this way, the copy manager 20a . . . 20 n may maintain different freeze timeouts for different sourcestorage locations 54 involved in copy relationships to allow writes toresume at different times for different source storage locations 54,depending on the freeze timeout times 58 defined in the copyrelationship information 50 for that storage location 54.

FIG. 3 illustrates further information maintained in a primary controlunit 4 a . . . 4 bn for use by the copy manager 20 a . . . 20 n,including one or more copy relationships 50, a minimum freeze timeouttime 60 indicating a consistency group minimum freeze timeout time usedacross all the copy relationships in the primary control units 4 a, 4 b.4 n that are managed by the consistency manager 16 in a singleconsistency group. The consistency manager 16 may provide the copymanager 20 a . . . 20 n in the primary control units 4 a . . . 4 n withthis value. The receive heartbeat interval 62 is an interval in whichthe copy manager 20 a . . . 20 n expects to receive a heartbeat signalfrom the consistency manager 16.

If the copy manager 20 a . . . 20 n does not receive the heartbeatsignal within the receive heartbeat interval 62, then the copy manager20 a . . . 20 n will initiate a freeze operation to quiesce furtherwrites. The freeze operation may be issued to those source-targetlocations, e.g., LSS pairs, registered in the sessions managed by thecopy manager 20 a . . . 20 n. In one embodiment, the copy manager 20 a .. . 20 n calculates the receive heartbeat interval 62 as a function ofthe consistency group minimum freeze timeout time 60, such that thereceive heartbeat interval 62 is less than the consistency group minimumfreeze timeout time 60. Using the consistency group minimum freezetimeout time to determine the receive heartbeat interval ensures thatany one primary control unit 4 a . . . 4 n would perform a freezeoperation before another primary control unit 4 a . . . 4 n would thawas a result of the expiration of that primary control unit's 4 a . . . 4n freeze timeout times. For instance, if a primary control unit 4 a . .. 4 n loses connection with the consistency manager 16, then there is aconcern that another primary control unit 4 a . . . 4 n may initiate afreeze operation as a result of some failure to copy writes to thetarget storage. If one primary control unit lost its connection with theconsistency manager 16, then it may continue to copy writes to thetarget storage after the primary control unit that performed the freezeoperation thaws. If this occurs, then target storage may includeinconsistent data because one primary control unit is writing dependentdata to the target side, while other primary control units thatperformed the freeze operation do not copy dependent data, resulting indata inconsistency at the target side. With the described embodiments,if the consistency manager 16 is assumed to send the heartbeat signalmore frequently than the receive heartbeat interval 62 and the receiveheartbeat interval 62 is less than the consistency group minimum freezetimeout time 60 across all primary control units 4 a . . . 4 n, than allprimary control units will freeze before any one of them thaws andpermits the application 14 writes to continue. This ensures that allprimary control units 4 a . . . 4 n will not send any further data tothe target after any other primary control unit thaws because allprimary control units involved in the consistency group will haveinitiated a freeze operation before any of them would thaw and permitwrites after a freeze.

In one embodiment, the receive heartbeat interval 62 may be calculatedby subtracting from the minimum freeze timeout time 60 the time it wouldtake the copy manager 20 a . . . 20 n to issue a freeze operation to allcopy relationships 50 maintained at the primary control unit 4 a . . . 4n, also known as a command runtime. This takes into account the commandruntime for the freeze to be implemented at all copy relationships 50,i.e., all LSSs, so that a primary control unit will issue a freezeoperation in enough time to allow the freeze to be implemented at all ofits copy relationships 50 before any other primary control unit can thawand allow the application 14 to continue writes to all primary controlunits 4 a . . . 4 n.

In one embodiment, the consistency manager 16 may maintain a consistencygroup comprised of one or more sessions. A session includessource/target pairs on one or more primary control units 4 a . . . 4 nand multiple sessions may include source/target pairs on the same ordifferent primary control units 4 a . . . 4 n. FIG. 4 illustrates anembodiment of session information 70 having: a session identifier (ID)72 and then one or more source/target pair instances for eachsource/target pair included in the session. For each source/target pairincluded in the session 72, the session information 70 includes theprimary control unit 74 a . . . 74 n and the source/target pair 76 a . .. 76 n in the primary control unit 74 a . . . 74 n included in thesession 72. The source/target pair 76 a . . . 76 n information mayidentify an LSS pair or other storage unit pairs in the primary 6 a . .. 6 n and secondary 12 a . . . 12 n storages.

FIG. 5 illustrates an embodiment of consistency group information 80 theconsistency manager 16 maintains for each consistency group beingmanaged. The consistency group information 80 includes a consistencygroup identifier (ID) 82; the one or more sessions 84 included in theconsistency group 82, where each session includes one or moresource/target pairs in one or more of the connected primary controlunits 4 a . . . 4 n; a consistency group minimum freeze timeout time 86indicating the minimum freeze timeout time across all primary controlunits 4 a, 4 b . . . 4 n including source/target pairs in theconsistency group 82; and a send heartbeat interval 88 calculated fromthe consistency group minimum freeze timeout time 86 at which theconsistency manager 16 sends heartbeat signals to the primary controlunits 4 a . . . 4 n managing source/target pairs in the consistencygroup 82.

FIG. 6 illustrates an embodiment of primary control unit information 90the consistency manager 16 maintains for each primary control unitincluding source/target pairs in a one consistency group 80. The primarycontrol unit information 90 indicates the control unit 92 and theminimum freeze timeout time 94 of the source/target pairs at thatcontrol unit 92.

FIG. 7 illustrates an embodiment of operations performed by theconsistency manager 16 and the copy manager 20 a . . . 20 n in theprimary control units 4 a . . . 4 n to exchange information to maintaindata consistency with respect to the freeze operation. The consistencymanager 16 performs the operations at blocks 100-110 and the copymanager 20 a . . . 20 n performs the operations at blocks 150-156. Uponthe consistency manager 16 initiating (at block 100) operations toregister source/target pairs from the primary control units 14 a . . .14 n in a consistency group 82 (FIG. 5), the consistency manager 16sends (at block 102) a registration to each connected primary controlunit 4 a . . . 4 n. Upon receiving this registration request, the copymanagers 20 a . . . 20 n at the primary control units 4 a . . . 4 ndetermine (at block 150) a minimum freeze timeout across allsource/target pairs (e.g., LSS pairs) to be added to the consistencygroup being registered. The minimum freeze timeout may be determinedacross all source/target pairs registered in the sessions managed by thecopy managers 20 a . . . 20 n. The copy manager 20 a . . . 20 n sends(at block 152) the consistency manager 16 the minimum freeze timeouttime at the primary control unit 4 a . . . 4 n and the source/targetpairs to register in the consistency group. The consistency manager 16saves (at block 104) the received information for source/target pairs(e.g., LSS pairs) for the primary control unit 4 a . . . 4 n and theminimum freeze timeout time 94 (FIG. 6) for the primary control unit 92with the primary control unit information 90.

Upon receiving registrations from all the primary control units 4 a . .. 4 n, the consistency manager 16 determines and saves (at block 106)the consistency group minimum freeze timeout time 86 (FIG. 5) as thedetermined minimum of the received control unit minimum freeze timeouttimes 94 (FIG. 6). The consistency manager 16 determines (at block 108)the send signal interval as a function of the consistency group minimumfreeze timeout time 86. In one embodiment, the send signal intervalcomprises a fraction of the receive heartbeat interval 62 (FIG. 3) usedby the copy managers 20 a . . . 20 n. In this way, the consistencymanager 16 sends the heartbeats to the primary control units 4 a . . . 4n at a higher frequency then the receive heartbeat interval to ensurethat the primary control units 4 a . . . 4 bn initiate freeze operationsbefore other primary control units 4 a . . . 4 n thaw and begin allowingapplication 24 writes. The consistency manager 16 sends (at block 110)the determined consistency group minimum freeze timeout time 86 to eachprimary control unit 4 a . . . 4 n including source/target pairs in theconsistency group 82 (FIG. 5) at issue.

Upon the copy manager 20 a . . . 20 n at the primary control unit 4 a .. . 4 n receiving (at block 154) the consistency group minimum freezetimeout 94, the copy manager 20 a . . . 20 n calculates (at block 156)the receive signal interval as a function of the consistency groupminimum freeze timeout time. As discussed, the calculated receiveheartbeat interval 62 may comprise the consistency group minimum freezetimeout time 86 less then the freeze command runtime. In an alternativeembodiment, the consistency manager 16 may calculate the receiveheartbeat interval 62 and then transmit that calculated value to thecopy managers 20 a . . . 20 n to use.

FIG. 8 illustrates an embodiment of consistency related operationsperformed by the consistency manager 16. The consistency manager 16initiates consistency operations (at block 200) and communicates (atblock 202) a heartbeat signal at the send heartbeat interval to theprimary control units 4 a . . . 4 n maintaining the source and targetpairs in the consistency group 82 (FIG. 5) being managed. Theconsistency manager 16 may send the heartbeat signals at the sendheartbeat interval 88 (FIG. 5) rate to all primary control units 74 a .. . 74 n (FIG. 4) in all sessions 84 (FIG. 5) identified in theconsistency group information 80 for the consistency group 82 beingmanaged. The consistency manager 16 may perform such operations formultiple consistency groups.

FIG. 9 illustrates an embodiment of operations performed by the copymanagers 20 a . . . 20 n to perform heartbeat signal management relatedoperations. Upon initiating (at block 220) heartbeat signal monitoringfrom the consistency manager 16, the copy manager 20 a . . . 20 n sets(at block 222) a timer for the receive heartbeat interval 62 (FIG. 3).If (at block 224) a signal (heartbeat) is received from the consistencymanager 16 before the timer expires, then control proceeds back to block222 to reset the timer and wait for the next heartbeat. Otherwise, if aheartbeat signal is not received from the copy manager 20 a . . . 20 nwithin the timer period (receive heartbeat interval 62), then the copymanager 20 a . . . 20 n initiates (at block 226) a freeze operation toblock further writes from applications 24 (FIG. 1) for all source/targetpairs managed by the primary control unit 4 a . . . 4 n. The freezeoperation may be sent to source/target pairs in the sessions registeredwith the copy manager 20 a . . . 20 n. In response to being blocked, theapplications 24 would stop sending writes to any primary control unit 4a . . . 4 n until the application 24 is notified that writes are allowedas part of the thaw operation. The copy manager 20 a . . . 20 n furthersends (at block 228) an interrupt to the consistency manager 16indicating a freeze. If the connection is available and the consistencymanager 16 receives this interrupt, then the consistency manager 16sends freeze commands to all the primary control units 4 a . . . 4 n inthe consistency group including the primary control unit from which theinterrupt was received. After commencing the freeze operation, the copymanager 20 a . . . 20 n starts (at block 230) the freeze timeout timerfor each source/target pair, where after a freeze timeout timer expires,the source (primary control unit) may initiate the thaw procedure andaccept writes for that source storage, e.g., LSS. After the freezethaws, the copy manager 20 a . . . 20 n would not copy writes over tothe target storage (secondary storage 12 a . . . 12 n), so that dataconsistency is maintained at the secondary (target) storages 12 a . . .12 n.

In a further embodiment, if a source/target pair is added or removed toa consistency group 82 (FIG. 5), then the consistency manager 16 mayperform the operations of FIG. 7 to recalculate the consistency groupminimum freeze timeout time 86 to allow adjustment of the send 88 (FIG.5) and receive 62 (FIG. 3) heartbeat intervals.

Described embodiments provide a technique to ensure that all primarycontrol units having source/target pairs in a consistency group will allinitiate freeze operations if one primary control unit initiates afreeze operation before any primary control unit thaws, or beginsaccepting writes after a freeze. With described embodiments, a primarycontrol unit maintaining communication with a consistency managerinitiates a freeze operation if the consistency manager sends a freezecommand in response to being notified of a freeze command by anothercontrol unit. Alternatively, if a primary control unit loses itsconnection with the consistency manager, then that primary control unitwould automatically begin a freeze operation if it did not receive aheartbeat signal from the consistency manager before any other primarycontrol unit could thaw after its freeze timeout time.

Additional Embodiment Details

The described operations may be implemented as a method, apparatus orarticle of manufacture using standard programming and/or engineeringtechniques to produce software, firmware, hardware, or any combinationthereof. The described operations may be implemented as code maintainedin a “computer readable medium”, where a processor may read and executethe code from the computer readable medium. A computer readable mediummay comprise media such as magnetic storage medium (e.g., hard diskdrives, floppy disks, tape, etc.), optical storage (CD-ROMs, DVDs,optical disks, etc.), volatile and non-volatile memory devices (e.g.,EEPROMs, ROMs, PROMs, RAMs, DRAMs, SRAMs, Flash Memory, firmware,programmable logic, etc.), etc. The code implementing the describedoperations may further be implemented in hardware logic (e.g., anintegrated circuit chip, Programmable Gate Array (PGA), ApplicationSpecific Integrated Circuit (ASIC), etc.). Still further, the codeimplementing the described operations may be implemented in“transmission signals”, where transmission signals may propagate throughspace or through a transmission media, such as an optical fiber, copperwire, etc. The transmission signals in which the code or logic isencoded may further comprise a wireless signal, satellite transmission,radio waves, infrared signals, Bluetooth, etc. The transmission signalsin which the code or logic is encoded is capable of being transmitted bya transmitting station and received by a receiving station, where thecode or logic encoded in the transmission signal may be decoded andstored in hardware or a computer readable medium at the receiving andtransmitting stations or devices. An “article of manufacture” comprisescomputer readable medium, hardware logic, and/or transmission signals inwhich code may be implemented. A device in which the code implementingthe described embodiments of operations is encoded may comprise acomputer readable medium or hardware logic. Of course, those skilled inthe art will recognize that many modifications may be made to thisconfiguration without departing from the scope of the present invention,and that the article of manufacture may comprise suitable informationbearing medium known in the art.

The terms “an embodiment”, “embodiment”, “embodiments”, “theembodiment”, “the embodiments”, “one or more embodiments”, “someembodiments”, and “one embodiment” mean “one or more (but not all)embodiments of the present invention(s)” unless expressly specifiedotherwise.

The terms “including”, “comprising”, “having” and variations thereofmean “including but not limited to”, unless expressly specifiedotherwise.

The enumerated listing of items does not imply that any or all of theitems are mutually exclusive, unless expressly specified otherwise.

The terms “a”, “an” and “the” mean “one or more”, unless expresslyspecified otherwise.

Devices that are in communication with each other need not be incontinuous communication with each other, unless expressly specifiedotherwise. In addition, devices that are in communication with eachother may communicate directly or indirectly through one or moreintermediaries.

A description of an embodiment with several components in communicationwith each other does not imply that all such components are required. Onthe contrary a variety of optional components are described toillustrate the wide variety of possible embodiments of the presentinvention.

Further, although process steps, method steps, algorithms or the likemay be described in a sequential order, such processes, methods andalgorithms may be configured to work in alternate orders. In otherwords, any sequence or order of steps that may be described does notnecessarily indicate a requirement that the steps be performed in thatorder. The steps of processes described herein may be performed in anyorder practical. Further, some steps may be performed simultaneously.

When a single device or article is described herein, it will be readilyapparent that more than one device/article (whether or not theycooperate) may be used in place of a single device/article. Similarly,where more than one device or article is described herein (whether ornot they cooperate), it will be readily apparent that a singledevice/article may be used in place of the more than one device orarticle or a different number of devices/articles may be used instead ofthe shown number of devices or programs. The functionality and/or thefeatures of a device may be alternatively embodied by one or more otherdevices which are not explicitly described as having suchfunctionality/features. Thus, other embodiments of the present inventionneed not include the device itself.

The illustrated operations of FIGS. 7, 8, and 9 show certain eventsoccurring in a certain order. In alternative embodiments, certainoperations may be performed in a different order, modified or removed.Moreover, steps may be added to the above described logic and stillconform to the described embodiments. Further, operations describedherein may occur sequentially or certain operations may be processed inparallel. Yet further, operations may be performed by a singleprocessing unit or by distributed processing units.

The foregoing description of various embodiments of the invention hasbeen presented for the purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed. Many modifications and variations are possible in lightof the above teaching. It is intended that the scope of the invention belimited not by this detailed description, but rather by the claimsappended hereto. The above specification, examples and data provide acomplete description of the manufacture and use of the composition ofthe invention. Since many embodiments of the invention can be madewithout departing from the spirit and scope of the invention, theinvention resides in the claims hereinafter appended.

1. A method, comprising: maintaining a copy relationship associating asource storage and target storage pair, wherein writes received at thesource storage are transferred to the target storage; determiningwhether a signal has been received from a system within a receive signalinterval; initiating a freeze operation to cease receiving writes at thesource storage from an application in response to determining that thesignal has not been received within the receive signal interval; andinitiating a thaw operation to continue receiving write operations atthe source storage from applications after a lapse of a freeze timeoutin response to the freeze operation, wherein after the thaw operation,received writes completed at the source storage are not transferred tothe target storage.
 2. The method of claim 1, wherein the systemcomprises a consistency manager, wherein the consistency manager managesdata consistency across multiple source and target storage pairsmaintained by different control units, further comprising: receiving,from the consistency manager, a minimum freeze timeout across the sourceand target storage pairs for which data consistency is maintained; anddetermining the receive signal interval from the received minimum freezetimeout.
 3. The method of claim 2, wherein the determined receive signalinterval comprises the received minimum freeze timeout less a time toimplement the freeze operation at the source and target storage pairs.4. The method of claim 2, further comprising: communicating the freezetimeout for the maintained copy relationship to the consistency manager.5. The method of claim 4, further comprising: using different freezetimeouts in the thaw operation for different managed copy relationshipsfor different source and target pairs, wherein the communicated freezetimeout comprises the minimum of the different freeze timeouts.
 6. Amethod, comprising: maintaining information on multiple source storageand target storage pairs maintained by control units, wherein thecontrol unit maintaining the pair copies writes to the source storage tothe target storage; determining freeze timeouts used by control unitsmaintaining the source and target pairs, wherein in response to a freezeoperation with respect to one source and target pair managed by onecontrol unit, the control unit blocks writes to the source storage,wherein the control unit initiates a thaw operation to continuereceiving write operations at the source storage after a lapse of thefreeze timeout for the source and target pair in response to the freezeoperation, and wherein after the thaw operation, received writescompleted at the source storage are not transferred to the targetstorage; determining a send signal interval based on the determinedfreeze timeouts; and communicating a signal at the send signal intervalto the control units maintaining the source and target pairs.
 7. Themethod of claim 6, wherein the determined freeze timeout for eachcontrol unit comprises a minimum freeze timeout used in the control unitfor the source and target pairs maintained by the control unit, andwherein the determined send signal interval is based on a minimum of thedetermined freeze timeouts for the control units.
 8. The method of claim7, wherein the determined send signal interval is less than the minimumof the determined freeze timeouts for the control units.
 9. The methodof claim 7, further comprising: sending the minimum of the determinedfreeze timeouts for the control units to each control unit.
 10. A systemin communication with a source storage, target store, and anapplication, comprising: a consistency manager; a copy relationshipassociating the source storage and the target storage pair, whereinwrites received at the source storage are transferred to the targetstorage; at least one control unit in communication with the consistencymanager enabled to perform operations, the operations comprising:determining whether a signal has been received from the consistencymanager within a receive signal interval; initiating a freeze operationto cease receiving writes at the source storage from the application inresponse to determining that the signal has not been received within thereceive signal interval; and initiating a thaw operation to continuereceiving write operations at the source storage from applications aftera lapse of a freeze timeout in response to the freeze operation, whereinafter the thaw operation, received writes completed at the sourcestorage are not transferred to the target storage.
 11. The system ofclaim 10, wherein the consistency manager manages data consistencyacross multiple source and target storage pairs maintained by differentcontrol units, wherein the control unit is further enabled to perform:receiving, from the consistency manager, a minimum freeze timeout acrossthe source and target storage pairs for which data consistency ismaintained; and determining the receive signal interval from thereceived minimum freeze timeout.
 12. The system of claim 10, wherein theconsistency manager is enabled to perform: determining freeze timeoutsused by control units maintaining the source and target pairs,determining a send signal interval based on the determined freezetimeouts; and communicating the signal at the send signal interval tothe control units maintaining the source and target pairs.
 13. Thesystem of claim 12, wherein the determined freeze timeout for eachcontrol unit comprises a minimum freeze timeout used in the control unitfor the source and target pairs maintained by the control unit, andwherein the determined send signal interval is based on a minimum of thedetermined freeze timeouts for the control units.
 14. An article ofmanufacture including code executed in a system in communication with aconsistency manager, a source storage, a target storage, and anapplication, wherein the code is executed to perform operations, theoperations comprising: maintaining a copy relationship associating thesource storage and the target storage pair, wherein writes received atthe source storage are transferred to the target storage; determiningwhether a signal has been received from the consistency manager within areceive signal interval; initiating a freeze operation to ceasereceiving writes at the source storage from the application in responseto determining that the signal has not been received within the receivesignal interval; and initiating a thaw operation to continue receivingwrite operations at the source storage from applications after a lapseof a freeze timeout in response to the freeze operation, wherein afterthe thaw operation, received writes completed at the source storage arenot transferred to the target storage.
 15. The article of manufacture ofclaim 14, wherein the consistency manager manages data consistencyacross multiple source and target storage pairs maintained by differentcontrol units, wherein the operations further comprise: receiving, fromthe consistency manager, a minimum freeze timeout across the source andtarget storage pairs for which data consistency is maintained; anddetermining the receive signal interval from the received minimum freezetimeout.
 16. The article of manufacture of claim 15, wherein thedetermined receive signal interval comprises the received minimum freezetimeout less a time to implement the freeze operation at the source andtarget storage pairs.
 17. The article of manufacture of claim 15,wherein the operations further comprise: communicating the freezetimeout for the maintained copy relationship to the consistency manager.18. The article of manufacture of claim 17, wherein the operationsfurther comprise: using different freeze timeouts in the thaw operationfor different managed copy relationships for different source and targetpairs, wherein the communicated freeze timeout comprises the minimum ofthe different freeze timeouts.
 19. An article of manufacture includingcode executed in a system in communication with a plurality of controlunits, source storages, target storages, and an application, wherein thecode is executed to perform operations, the operations comprising:maintaining information on multiple source storage and target storagepairs maintained by the control units, wherein the control unitsmaintaining the pair copies writes to the source storage to the targetstorage; determining freeze timeouts used by the control unitsmaintaining the source and target pairs, wherein in response to a freezeoperation with respect to one source and target pair managed by onecontrol unit, wherein the control unit blocks writes to the sourcestorage, wherein the control unit initiates a thaw operation to continuereceiving write operations at the source storage after a lapse of thefreeze timeout for the source and target pair in response to the freezeoperation, and wherein after the thaw operation, received writescompleted at the source storage are not transferred to the targetstorage; determining a send signal interval based on the determinedfreeze timeouts; and communicating a signal at the send signal intervalto the control units maintaining the source and target pairs.
 20. Thearticle of manufacture of claim 19, wherein the determined freezetimeout for each control unit comprises a minimum freeze timeout used inthe control unit for the source and target pairs maintained by thecontrol unit, and wherein the determined send signal interval is basedon a minimum of the determined freeze timeouts for the control units.21. The article of manufacture of claim 20, wherein the determined sendsignal interval is less than the minimum of the determined freezetimeouts for the control units.
 22. The article of manufacture of claim20, wherein the operations further comprise: sending the minimum of thedetermined freeze timeouts for the control units to each control unit.